moving cert&key loading code to p11
Adjust cert/key loading in p11.
 - simplify loading key/cert by moving the (shared) slot lookup code
   to PKCS11_find_by_slot_id
 - rename PKCS11_RSA_CRYPTO_EX to PKCS11_CRYPTO_EX
 - PIN handling is now part of p11 (get_pin procedure is still external)
 - adds exports for PKCS11_{load_key,load_cert,PIN_alloc,PIN_clear,PIN_dup)
releasing a slot shall not release all sessions to this slot.